According to Dennard scaling, voltage and current should be proportional to the linear dimensions of a transistor, and power consumption (the product of voltage and current) should be proportional to the area of a transistor. As the sizes of transistors continue to shrink, the number of transistors that can fit into the same area of a chip has grown exponentially. Thus, it has been predicted that the computing performance per watt can also grow exponentially. However, Dennard scaling appears to have broken down in the last decade. Even though the size of transistors continues to shrink, the per watt computing performance has not improved at the same rate. There are various reasons for the breakdown of Dennard scaling. One of the reasons is that at small sizes current leakage can cause a chip to heat up which increases energy costs and the risk of thermal runaway. To prevent thermal runaway, a portion of the silicon on the chip cannot be powered-on at the nominal operating voltage for a given thermal design power (TDP) constraint. This phenomenon, referred to as “dark silicon,” significantly constraints the per watt computing performance in modern processors.
The breakdown of Dennard scaling has prompted some chip manufacturers to resort to multicore processor designs. However, even multicore processors have encountered the same “dark silicon” problem. Depending on the processor architecture, cooling technology, and application workloads, the amount of dark silicon may exceed 50%. Thus, there is a need to improve energy and computing efficiency in modern computer systems.